1. Field of the Invention
This invention relates to portable power tools and more specifically, to powered drilling apparatus of the type that executes an automatic drilling cycle consisting of: (1) clamping the drilling apparatus to the workpiece and, in most instances, to a template or jig that positions the drill spindle relative to the workpiece; (2) advancing or feeding a rotating tool bit (e.g., twist drill, countersink, or combined twist drill-countersink) to effect the desired machining operations; (3) withdrawing the tool bit from the machined opening and (4) releasing the clamping mechanism that secures the drilling apparatus to the workpiece.
2. Description of the Prior Art
Pneumatically operated, self-colleting, power feed drill motors of the above-mentioned type are presently utilized in the manufacture of various structural assemblies, being of particular importance in the drilling and countersinking of precision holes during the fabrication, maintenance, and repair of airframe assemblies, including conventional transport aircraft and space vehicles. As known to those skilled in the art, such drill motors are generally clamped to the workpiece by means of a collet foot or base assembly that extends from the forward portion of the drill motor. An expansible collet that is alternatively located at a fixed position in the base assembly or mounted therein so as to be an adjustable distance from the position at which a twist drill (or other tool bit) is to contact the workpiece is operated by a mandrel that extends through the collet. The mandrel is in turn operated by an axially translatable drawbar that is connected to the piston of a pneumatic cylinder so that the collet expands and contracts as the drawbar is moved respectively away from and toward the base assembly.
In the most commonly employed manufacturing method, a thin metal template having openings that define the desired hole pattern is placed against a workpiece such as, for example, aircraft skin panels that are temporarily held in position against structural members of an aircraft wing, fuselage, or other such assembly or subassembly. The drill motor is then positioned so that the base assembly abuts the template with the collet extending through an opening in the template and through a previously-drilled hole in the workpiece. A shoulder or boss that circumferentially surrounds an opening through which the drill or tool bit will emerge is positioned within a second opening of the template. The power tool is then activated by squeezing a conventional trigger control on a pistol-grip type handle that extends from the drill motor and the pneumatic cylinder retracts the drawbar and mandrel so that the collet expands in the opening of the workpiece. This action insures that the base assembly remains against the workpiece and clamps the drill motor in the proper position. A pneumatic motor is automatically activated to drive the drill spindle via reduction gears and a second pneumatic cylinder drives the spindle so as to feed the rotating tool bit into the workpiece. During this portion of the sequence, a hydraulic control circuit maintains the feed rate at or within desired limits. When the outward extension of the spindle reaches a preset limit or stop position, the sequence is reversed to retract the tool bit and then move the mandrel away from the base assembly to allow the collet to contract so that the drill motor can be repositioned in a different opening of the template.
Although satisfactory in some situations, prior art drill motors of the above-described type exhibit several disadvantages and drawbacks. First, such drill motors are relatively large and heavy and because of such size and weight often cannot be utilized in limited quarters. Secondly, drill motors of the above-described type have remained a rather specialized tool with a single type of drill motor accommodating only a rather limited range of drilling depths, drill diameters, drilling speed and feed rate. Moreover, although an expansible collet that replaces the above-discussed base assembly adapts some prior art drill motors for use with precision drill jigs, prior art devices have not been adaptable to other manufacturing situations.
Another drawback and disadvantage of the prior art apparatus is that hydraulic pressure for the hydraulic control system has generally been provided by a gear-type pump that is driven by the same pneumatic motor that drives the drill spindle as well as the system feed and clamp cylinders. Because of this, the clamp-up force and feed thrust provided by prior art drill motors have not been as great as possible. Moreover, the gear-type pump of such a prior art unit is constantly driven throughout the entire period of time that the drill motor is actuated. Thus, both the pump and the pneumatically-driven motor are subject to substantial wear and maintenance. Moreover, driving the gear pump during the period in which the workpiece is being drilled or machined in another manner can unnecessarily limit the torque produced by the drill motor. This can be especially important when a drill breaks through the workpiece, since stalling is then more likely to occur. In some cases, if the drill motor stalls, hydraulic power may terminate and allow the drill motor to unclamp from the workpiece. Such unclamping can assert bending loads that break the drill bit and/or damage the hole that has been machined in the workpiece.
Additionally, the hydraulic control circuits utilized in the prior art drill motors to automatically sequence the tool through the steps of “clamp-up,” drill thrust, drill retraction and unclamping, are relatively complex and are not as reliable as is often desired. In some cases, the requirements of the pneumatic drill motor reduce the clamp-up and thrust forces to a degree that results in hole elongation, drill breakage, or other damage. Prior art drill motors have limited hole-making accuracy because the forward and rear drill spindle support bearings are not rigidly attached in an essentially one-piece housing with the result that the rear bearing slides relative to the front bearing while the machine is drilling. This sliding requires mechanical clearance, which when combined with the resistance developed by the hydraulic feed control mechanism which is not in line with the feed force, causes the rear spindle bearings to move off-axis from the centerline of the hole to be drilled. In addition, the forward spindle bearing is a plain bearing which needs clearance to prevent seizure of the drill spindle. This clearance also limits the potential accuracy of the drilled hole, and allows fine drilling chips into the clearance between the spindle and plain bearing. The chips cause accelerated wear, reducing hole accuracy and increasing tool maintenance.
One United States patent which attempted to address the shortcomings of the prior art discussed above is U.S. Pat. No. 4,594,030 issued to Weigel. This particular embodiment had advantage over the drills in the prior art, but had difficulties due to its design. One of the major drawbacks of Weigel was that hydraulic pump design problems arose with its piston-type shuttle pump resulting in air leaking in the oil used in that pump, resulting in air bubbles in the hydraulic system causing failure ultimately. Means to bleed air from the system were lacking. Further, the drill used a plain spindle bearing which had difficulty with its ability to be lubricated. Given the allowed clearance for the disclosed bearing, it was impossible to get a proper oil film on it to facilitate lubrication. If more clearance in the drive system was built in to allow the bearings to be coated with oil, thereby preventing excess heat and eventually failure, accuracy of the drilled hole was sacrificed. This one defect rendered the unit of that patent problematic during operation.
Other problems with Weigel included its clamping system wherein the unit is clamped to the workpiece prior to drilling. The many linkages involved resulted in binding problems which were significant. The linkage was not strong enough in the high clamp forces created by the hydraulic clamp cylinder which assured a tight clamping to the workpiece. The links in the Weigel system tended to flex and eventually jam. Other shortcomings of the system of the Weigel patent also existed including the force required to operate the drill trigger, and the fact that the collet of the clamping system pulled only with a center pin, resulting in less strength and rigidity in that system than what was desired.
Accordingly, it is an object of this invention to provide a drill for drilling precise and accurate holes on a workpiece, such as an airplane fuselage, which functions efficiently, smoothly, and consistently.
Another object of the present invention is to provide a drill that is adaptable to a large number of drill bit sizes and drilling requirements specified in the aircraft industry.
Yet another object of the present invention is to provide a drill apparatus that can be used to drill holes in applications that require accuracy.